In a conventional batch type annealing process for producing a cold rolled steel strip useful for usual drawing processes, a cold rolled steel strip is coiled tightly or loosely, and the coil is placed in a box-shaped furnace and annealed therein at a desired temperature. However, this conventional batch type annealing process needs several days to complete the entire process thereof. That is, the efficiency of the process is very poor. In order to eliminate the above-mentioned disadvantage of the batch type annealing process, various continuous annealing processes which can be completed within about ten minutes, were attempted. Some of them are practically utilized in industry.
The conventional continuous annealing process are more advantageous than the conventional batch type annealing process in the following points.
1. The cost of the annealing apparatus is remarkably low.
2. The production cost is low, because the process can be effected by a reduced number of operations with a reduced consumption of energy at an enhanced yield.
3. The quality in appearance and surface quality, for example, flatness or shape, of the resultant product is superior.
4. The production can be completed at a high speed and the product can be quickly delivered.
Therefore, the continuous annealing processes are becoming an important area of investment in the steel-making industry.
However, in order to enhance the efficiency of the investment, it is required that the continuous annealing apparatus can exhibit an enhanced capacity and is capable of being applied to the production of not only the mild steel sheet (usable for the drawing process) but also, the high strength steel strip. Demand for a high tensile strength steel strip has recently been increasing. In order to satisfy the above-mentioned requirements, the conventional continuous annealing process and apparatus must be free from several problems. The problems will be explained below.
In a conventional continuous annealing process, the cold rolled steel strip which has been heated to a predetermined annealing temperature and, then, held at the annealing temperature for a predetermined time period, is usually cooled to a predetermined temperature by jetting an inert cooling gas toward the steel strip. The cooling gas jetting method exhibits the following advantages.
1. It is easy to stop the cooling procedure when the steel strip reaches a predetermined decreased temperature, for example, a predetermined overaging temperature. Therefore, when the cooled steel strip is subjected to an overaging procedure, it is unnecessary to heat the steel strip to the overaging temperature.
2. Since the cooling procedure is carried out by using an inert cooling gas, the surface of the steel strip is not oxidized, so as to maintain a bright surface. Therefore, it is not necessary to subject the cooled steel strip to a removal procedure of an oxide layer from the steel strip.
3. The cooling procedure does not cause the steel strip to be deformed. Therefore, the resultant cooled steel strip always has a satisfactory shape.
However, the cooling gas jetting method causes the cooling rate on the steel strip to be low, for example, 10.degree. C./sec. or less. Therefore, the steel sheet must be overaged over a long period of time, and the annealing equipment including the cooling and overaging apparatus must be very long which is expensive. Also, in the case where a high tensile strength steel strip having a dual-phase structure is annealed and cooled by the cooling gas jetting method, it is necessary that the steel strip is produced from a steel material containing a relatively large amount of an expensive alloy element, for example, manganese. In this case, the resultant product becomes expensive.
In another conventional continuous annealing process, the cold rolled steel strip which has been held at a predetermined annealing temperature for a predetermined time period, is cooled by immersing the steel strip into water, that is, a water-quenching method. In this method, since the cooling water is directly brought into contact with the steel strip, the cooling rate of the steel strip is high, for example, 10.sup.3 .degree. C./sec. or more. This rapid cooling causes precipitation of oversaturated solid-soluted carbon in the steel sheet to be accelerated. However, since the cooling rate is too high, the temperature of the steel sheet rapidly reaches the same level of the cooling water. Therefore, it is difficult to stop the cooling procedure while the temperature of the steel sheet is still higher than that of the cooling water. When the annealed steel sheet is cooled to the same temperature as that of the cooling water, it is necessary to re-heat the steel sheet to a desired overaging temperature. This heating cost causes the price of the resultant product to be increased.
The same disadvantages as those of the steel sheet occur on the high tensile strength steel strip having a dual phase-structure. That is, when the high tensile strength steel strip is annealed and then, cooled by immersing it in cooling water, the excessively high cooling rate of the steel strip results in such an undesirable phenomenon that the solid-soluted carbon in the steel strip is quenched. Therefore, it is necessary to re-heat the cooled steel strip to the desired overaging temperature, for example, about 250.degree. C. This re-heating procedure causes the cost of producing the high tensile strength steel strip to be increased. When the cold rolled steel sheet or high tensile strength steel strip is re-heated to the overaging temperature, the solid-soluted carbon precipitates in the form of carbides into the ferrite crystal grains. This phenomenon causes the ductility of the product to be degraded, and, therefore, the product to become useless. Also, the necessity of the re-heating procedure causes the necessity of addition of a re-heating apparatus which is very expensive to the annealing-overaging equipment. Therefore, the annealing-overaging processing time becomes long and the annealing-overaging equipment becomes costly.
In another cooling method, the annealing steel strip is directly immersed in a molten salt bath. In this method, since the melted salt has a great cooling capacity, it is possible to rapidly cool the steel strip to a desired temperature by maintaining the temperature of the melted salt at a desired temperature. However, this method is disadvantageous in that the cooling rate is not variable over the cooling procedure. That is, it is impossible to gradually cool the steel strip in the initial stage of the cooling procedure and, then, rapidly cool it in the final stage of the cooling procedure. This disadvantage sometimes causes the cooled steel strip to be deformed. Also, the melted salt method is disadvantageous such that when a portion of the melted salt adheres to the surface of the steel strip, the adhered portion of the melted salt is transferred onto surfaces of rollers in an overaging apparatus and accumulates thereon. The accumulated salt on the overaging rollers causes the quality of the resultant overaged steel strip to be degraded. Also, it is difficult to remove the adhered salt from the strip surface.
In another cooling method, streams of a cooling liquid were sprayed onto the surface of the steel strip. When the cooling liquid used has a great cooling capacity and the flow rate of the cooling liquid to be jetted is controlled, it is possible to rapidly cool the steel strip down to a desired temperature at a desired cooling rate. However, in the cooling liquid-spraying procedure, when the flow rate of the cooling liquid is decreased, sometimes, the streams of the sprayed cooling liquid cannot reach the surface of the steel strip. In this case, the steel strip is not cooled, but deformed.
In still another cooling method, a mixture of a cooling gas and an atomized cooling liquid is jetted onto the surface of the steel strip through a jetting nozzle. This method is valuable and disclosed in Japanese Patent Application Publication No. 53-15803(1978). In this method, the atomized cooling liquid having a relatively large cooling capacity is carried by the cooling gas stream having a relatively small cooling capacity. Therefore, it is possible to vary the cooling capacity of the cooling liquid-gas mixture stream by varying the amount of the cooling liquid to be contained in the mixture. That is, it is possible to vary the cooling rate of the steel strip by controlling the amount of the cooling liquid.
However, when the atomized cooling liquid is preliminarily mixed with the cooling gas and then, the resultant cooling liquid-gas mixture is jetted through a jetting nozzle, the fine particles of atomized cooling liquid are aggregated together to form large drops of the liquid in the mixture before the mixture reaches the surface of the steel strip. In this case, it is difficult to uniformly cool the steel strip at a high cooling rate.